1. Field of the Invention
The present invention relates to an improved process for the preparation of hexene-1,6-dioic acids, and, in particular, hex-3-ene-1,6-dioic acid. Hex-3-ene-1,6-dioic acid is a valuable intermediate that can facilely be hydrogenerated into adipic acid.
2. Description of the Prior Art
Adipic acid, one of the starting materials for the production of nylon 66, is currently produced in vast amounts per annum. By reason of this fact alone, any new route for the synthesis of this diacid and/or derivatives thereof would be of fundamental interest to this art.
Published French Patent Application No. 89/06,019 describes a process for the preparation of such diacids by contacting but-2-ene-1,4-diol with carbon monoxide in the presence of a palladium-based catalyst, at elevated temperature and under a pressure higher than atmospheric pressure, and wherein the reaction is also carried out in the presence of at least one quaternary onium chloride of a Group VB element of the Periodic Table selected from nitrogen and phosphorus, such element being tetracoordinated to carbon atoms, with the proviso that the nitrogen may be coordinated to two pentavalent phosphorus atoms.
This reaction can be carried out in N-methylpyrrolid-2-one.
The process of the '019 application, which provides appreciable results both in respect of its activity and in respect of its selectivity for a straight-chain dicarbonylated compound, however, presents the disadvantage of requiring the presence of at least one quaternary onium halide as indicated above. These onium compounds are promoters which are relatively expensive or not readily available and are susceptible to degradation during prolonged use.
Thus, need exists in this art for an alternate process that does not require a halogenated organic promoter, and particularly one wherein all or part of such organic promoter may be replaced by an inorganic halogenated promoter which is more readily available and relatively more stable over prolonged use.